This invention relates to a process for producing hydrocarbons useful as fuel, such as aviation fuel, from renewable feedstocks such as the glycerides and free fatty acids found in materials such as plant oils, fish oils, animal fats, and greases. The process involves hydrogenation, decarboxylation, decarbonylation, and/or hydrodeoxygenation, hydroisomerization, and selective hydrocracking occurring in a single reaction stage. The single reaction stage may include multiple reactor beds or multiple reaction vessels. The selective hydrocracking reaction optimally provides one hydrocracking event per molecule. An integrated reforming step or a steam reforming step may be optionally employed to generate at least a portion of the hydrogen that is chemically consumed in reaction stage.
As the demand for fuels such as aviation fuel increases worldwide there is increasing interest in sources other than petroleum crude oil for producing the fuel. One such source is what has been termed renewable feedstocks. These renewable feedstocks include, but are not limited to, plant oils such as corn, jatropha, camelina, rapeseed, canola, soybean and algal oils, animal fats such as tallow, fish oils and various waste streams such as yellow and brown greases and sewage sludge. The common feature of these feedstocks is that they are composed of mono- di- and tri-glycerides, free fatty acids (FAA). Another class of compounds appropriate for these processes fatty acid alkyl esters (FAAE), such as fatty acid methyl ester (FAME) or fatty acid ethyl ester (FAEE). These types of compounds contain aliphatic carbon chains generally having from about 8 to about 24 carbon atoms. The aliphatic carbon chains in the glycerides, FFAs, or FAAEs can be saturated or mono-, di- or poly-unsaturated. Most of the glycerides in the renewable feed stocks will be triglycerides, but some of the glycerides in the renewable feedstock may be monoglycerides or diglycerides. The monoglycerides and diglycerides can be processed along with the triglycerides.
There are reports disclosing the production of hydrocarbons from oils. For example, U.S. Pat. No. 4,300,009 discloses the use of crystalline aluminosilicate zeolites to convert plant oils such as corn oil to hydrocarbons such as gasoline and chemicals such as para-xylene. U.S. Pat. No. 4,992,605 discloses the production of hydrocarbon products in the diesel boiling range by hydroprocessing vegetable oils such as canola or sunflower oil. Finally, US 2004/0230085 A1 discloses a process for treating a hydrocarbon component of biological origin by hydrodeoxygenation followed by isomerization.
A process which comprises a single reaction zone to hydrogenate, deoxygenate, isomerize and selectively hydrocrack a renewable feedstock, in order to generate a hydrocarbon product having paraffins with boiling points in the aviation fuel range is provided herein. The hydrocarbon product is useful as an aviation fuel or an aviation fuel blending component. Simply deoxygenating the renewable feedstock in a hydrogen environment in the presence of a hydrotreating catalyst results in straight chain paraffins having chain-lengths similar to, or slightly shorter than, the fatty acid composition of the feedstock. With many feedstocks, this approach may result in a product having an appropriate carbon chain length for a diesel fuel, but not meeting the specifications for an aviation fuel. The selective hydrocracking reaction reduces the carbon chain length to maximize the selectivity to aviation fuel range paraffins while minimizing lower molecular weight products. Isomerization allows for aviation fuel specifications, such as freeze point, to be met. Successfully conducting the required reactions in a single reaction zone can result in a lower capital and operating cost structure. An optional reforming step or steam reforming step may be included to generate the hydrogen needed in the hydrogenation, deoxygenation, and the hydrocracking reactions. In one embodiment, a portion of the effluent of the reaction zone is recycled to the reaction zone. The volume ratio of recycle hydrocarbon to feedstock ranges from about 0.1:1 to about 8:1 and provides a mechanism to limit the reaction zone temperature rise, increase the hydrogen solubility and more uniformly distribute the heat of reaction in the reaction mixture. As a result of the recycle, some embodiments may use less processing equipment, less excess hydrogen, a lower operating pressure, less utilities, or any combination of the above. In another embodiment, hydrogen may be supplied by integrating the process into an existing hydrogen generating facility such as a refinery.